This invention relates to an AC-to-DC power converter apparatus.
Heretofore, various proposals have been made on apparatus employing full wave bridge circuits for obtaining variable DC power from AC power sources or for converting DC to AC power and they have been put to practical use. Moreover, single-phase or polyphase AC is employed according to the use; although the apparatus which employs polyphase AC is more complicated technically as an apparatus than that which employs single-phase AC, there are many problems in common. A three-phase AC apparatus will be described below and based upon such description, an apparatus employing a single-phase AC will be readily understood.
A full wave bridge circuit is generally made up of thyristors, wherein the ignition phase angles of a group of thyristors connected to the positive arm of the circuit and another group of thyristors connected to its negative arm are made equal, the circuit being used to control output voltage by making the phase angle variable.
However, the disadvantage of this method is that the power-factor is worsened, while the ripple component is increased, in regions where DC output voltage is low.
As the method of improving the disadvantage in such a low output voltage region as this, the present applicant previously proposed U.S. Pat. No. 4,245,293. In the above method, the ignition phase angles of the groups of thyristors on the positive and negative sides in the low output voltage region are defined as being different in order to make improvements by causing a short-circuiting mode between both the groups of thyristors.
However, as further improvements in the powerfactor has been desired recently, the following methods have been proposed.
(1) "A Pulsewidth Controlled AC-to-DC Converter to Improve Power Factor and Waveform of AC Line Current" (IEEE TRANSACTION ON INDUSTRY APPLICATIONS, VOL. IA-15, No. 6, NOVEMBER/DECEMBER, 1979)
(2) British Patent Official Gazette No. 2076233 (corresponding to U.S. Pat. No. 4,361,886).
In either case, the mode of supplying power to an AC load by connecting controllable switching means with the function of breaking current to the positive arm of a full wave bridge circuit to control the switching means and by rectifying an AC power source and the mode of causing the current flowing through the DC load to flow back at the time of breaking current in the power supplying mode are repeated and AC-to-DC power conversion may be carried out.
In this method, it is possible to make the output voltage variable by making variable the period (chopping pulsewidth) of the above power supplying mode and, because the relation between the AC supply source voltage and current is maintained roughly in the same phase, the power-factor is allowed to be close to 1. Due to the development of elements having large capacitance recently such as gate turn-off thyristors (here after referred to as "GTO elements") and transistors, this method is attracting attention.
However, it has been made clear that, because the power supplying mode and flowback mode are repeated by forcing the current being supplied to be cut off using the controllable means with the function of breaking the current in this method, the switching means may be destroyed then and made uncontrollable.